The present invention generally relates to communication receivers, and particularly relates to mixer circuits for use in communication receivers.
Communication receiver circuits operate in challenging performance environments. For example, in Frequency Division Multiple Access (FDMA) systems, such as those configured according to Wideband CDMA (WCDMA) standards, the receiver operates concurrently with the local transmitter and must exhibit good transmit frequency rejection. Casting the self-interference problem in numeric terms, a WCDMA transmitter at +25 dBm results in a −25 dBm interferer signal into the WCDMA receiver, assuming receive/transmit duplexer attenuation of 50 dB. If receiver performance criteria require no more than −108 dBm of static interference, then the receiver's second-order intercept point (IP2) must be ≧+49 dBm for the rectified transmit spectrum to be below the −108 dBm limit.
Placing a suppression filter between the receiver's low-noise amplifier and mixer circuit represents one approach to suppressing interference, including “leakage” interference from the local transmitter. However, the proximity of transmit and receive frequencies, sometimes referred to as the “duplex distance,” requires relatively stringent filter performance characteristics on these suppression filters. Indeed, the required filter sharpness leads to the use of Surface Acoustic Wave (SAW) filters, or other high-performance filter circuits, which prevent easy integration with on-chip transceiver circuits.